The present invention generally relates to a method and an apparatus for mutually converting different signaling systems between two transmission devices using the North American Standards (NAS or T1 signals) and the European Conference of Posts Telecommunications Administrations (CEPT or E1 signals) respectively.
Due to the growing application of digital transmission techniques, it is required to establish transmission layers for implementing systematical transmission lines. Studies of international standardization of digital hierarchy systems have been promoted by CCITT etc. from the nineteen seventies.
Recently, internationally recommended digital hierarchy of the CEPT (referred to as E1 signals hereinafter) based on the transmission rate of 2.048 Mb/s and NAS (referred to as T1 signals hereinafter) based on the transmission rate of 1.544 Mb/s are being utilized generally.
Also international interworking hierarchy for digital connection between nations utilizing different digital hierarchy from each other is recommended by CCITT.
Accordingly, an interfacing function of two different signaling systems is required between different digital hierarchies.
Representative examples of these signaling systems are the 1.544 Mb/s signaling systems.
The differences between these two signaling systems are as follows;
In the T1 signals, one multi-frame consists of either 12 frames or 24 frames, with each frame consisting of 1 framing bit and 24 channels, each of which consists of 8 bits. In the multi-frame consisting of 12 frames, the 8th bit of each channel of the 6th and 12th frames of one multi-frame is used as the signaling bit, while in the multi-frame consisting of 24 frames, the 8th bit of each channel of the 6th, 12th, 18th and 24th frames of one multi-frame is used as the signaling bit.
The line signal comprises forward signaling bit (af) transmitted from the outgoing trunk and backward signaling bit (ab) transmitted from the incoming trunk of the exchange, the bit (af) representing hook-off/hook-on states at the calling side, and the bit (ab) representing hook-off/hook-on states at receiving subscriber, or being used as a seizure acknowledge signal at the incoming trunk for the seizure of the trunk.
In the E1 signals, one multi-frame consists of 16 frames, each frame consisting of 32 channels, each of which consists of 8 bits. The 8 bits of channel No. 16 of each frame except 0th frame are used as signaling bits of two channels by 4 bits per one channel. The line signals in this format include forward signaling bits (af, bf) and backward signaling bits (ab, bb), like the T1 signals. Bit of represents hook-off/hook-on states at calling side; the bit bf represents abnormal state at calling side; the bit, ab, represents hook-off/hook-on states at receiving side, and the bit bb represents the state of receiving side.
As described above, when interworking different signaling systems, a converter for mutually converting different signaling systems is connected between the two exchanges using different signaling systems.
FIG. 1 shows the two cases of conversion performed by such a signaling converter. In FIG. 1, E1 exchange (1) performs transmitting/receiving through outgoing trunks (11) and incoming trunks (12). Also, the T1 exchange (3) performs transmitting/receiving in the same manner as the E1 exchange (1), but these exchanges use different signaling systems. Therefore, a converter (2) for mutually converting different signalling systems must be connected between these two exchanges.
(E1 exchange and T1 exchange are referred to as the digital exchange or equivalent of it's digital trunk operating with T1 signal and E1 signal, respectively.)
The description of operation of such a converter is as follows. Firstly, in the case that E1 exchange (1) operates with the outgoing trunks (11) and T1 exchange (3) operates with the incoming trunks (31), forward signaling information (af, bf) of the E1 signals is converted into forward signaling information (af) of the T1 signals in a converter (2) for mutually converting different signaling systems and then transmitted to the incoming trunks (31) of the T1 exchange (3). Whereas backward signaling information (ab) of T1 signals is converted into backward signaling information (ab, bb) of E1 signals in the converter (2) and then transmitted to the incoming trunks (11) of the E1 exchange (1). On the other hand, in the case that the E1 exchange (1) operates with the incoming trunks (12) and the T1 exchange (3) operates with the outgoing trunks (32), the forward signaling information (af) of the T1 signals is converted into the forward signaling information (af, bf) of the E1 signals and then transmitted to the E1 exchange (1); and the backward signaling information (ab, bb) of the E1 signals is converted into the backward signaling information (ab) of the T1 signals and then is transmitted to the T1 exchange (3).
At this time, the two cases of conversion which are shown by dash-line (4), (5), in FIG. 1 are given to each trunk at T1 exchange (3), so that, if trunks of T1 exchange (3) have the type (4), channels of them become incoming channels and channels of trunks at E1 exchange (1) become outgoing channels, whereas if trunks of T1 exchange (3) have the type (5), channels of them become outgoing channels and channels at E1 exchange (3) become incoming channels. Therefore, in accordance with the kinds of signaling method (R2 signaling or loop signaling), signaling conversion functions can be performed separatively in the converter (2).
In the present situations, mechanical exchanges such as ST EMD, etc. and electronic exchanges such as M10CN, No. 1A, etc. are utilized as local exchanges, and toll exchanges such as No. 4, AXE-10, etc. are used, too. These exchanges must be interconnected and interworking, always; thus between any two exchanges, as the case may be loop signaling, EMD signaling, or R2 signaling method, etc. are utilized.
Also, loop signaling method or R2 signaling method is utilized in digital trunks, as well as in transmission system using E1 signaling systems to be introduced in the future.